1. Field of the Invention
The present invention relates to an improvement of a ranging function provided in a redundant optical access system. The invention is applicable to a redundant optical access system for constructing an access system such as a PON (Passive Optical Network) and the like.
2. Description of the Related Art
Hitherto, FTTx (Fiber To The x) has been known as an optical access network for providing communication services such as Internet, IP (Internet Protocol) telephone, distribution of video and the like. As the FTTx, there have been known FTTH (Fiber To The Home), FTTC (Fiber To The Curb), FTTN (Fiber To The Node), FTTP (Fiber To The Premises) and the like for example.
The PON is also known as a subscriber access technology for realizing the FTTx at low cost. As the PON, there have been known ATM-PON (Asynchronous Transfer Mode-PON: technology standardized by ITU-T G.983.1 and G983.2), B-PON (Broadband-PON: technology standardized by ITU-T G.983.3), G-PON (Gigabit-PON: technology standardized by ITU-T G.984) and GE-PON (Gigabit Ethernet (Registered Mark)-PON: technology standardized by IEEE802.3ah).
FIG. 3 is a conceptual diagram showing a topology of the PON. As shown in FIG. 3, an OLT (Optical Line Terminal: subscriber station unit) 301 accommodates n ONUs (Optical Network Unit: optical line terminal unit) 305-1, . . . , 305-n via a splitter 302 and optical fibers 303 and 304-1, . . . , 304-n. The OLT 301 and the ONUs 305-1, . . . , 305-n use different frequencies for downstream and upstream communications. Therefore, it is possible to carry out the downstream and upstream communications in parallel.
In the downstream communication, the OLT 301 receives IP packets addressed to the ONUs 305-1, . . . , 305-n from a host network 311. Then, the OLT 301 generates time division multiplexed downstream signals from these IP packets. The downstream signal may contain communication data D1, . . . , Dn addressed to each of the ONUs 305-1, . . . , 305-n. This downstream signal is outputted from the OLT 301 and arrives at the splitter 302 via the optical fiber 303. The splitter 302 outputs the same downstream signal to each of the optical fibers 304-1, . . . , 304-n. Receiving the downstream signal from the corresponding optical fibers 304-1, . . . , 304-n, the ONUs 305-1, . . . , 305-n extract the IP packets D1, . . . , Dn addressed to own, to convert into communication data and sends to corresponding communication terminals 312 (e.g., a personal computer, an IP telephone and the like). It is noted that the OLT 301 transmits the communication data D1, . . . , Dn by encrypting them in order to assure confidentiality of the communication (i.e., so that the ONUs other than the addressed ONU cannot decode the communication data D1, . . . , Dn).
On the other hand, during the upstream communication, the ONUs 305-1, . . . , 305-n receive communication data U1, . . . , Un from the corresponding communication terminals 312. The communication data U1, . . . , Un are outputted at timing set per each ONU 305-1, . . . , 305-N. The communication data U1, . . . , Un arrive at the splitter 302 via the optical fibers 304-1, . . . , 304-n. The splitter 302 superimposes the communication data U1, . . . , Un as they are. At this time, a multiplexed upstream signal may be generated by adequately setting the timing for outputting the communication data U1, . . . , Un from each of the ONUs 305-1, . . . , 305-n, because the splitter 302 superimposes the communication data. The upstream signal is outputted from the splitter 302 and arrives at the OLT 301 via the optical fiber 303. The OLT 301 multiplies and separates the upstream signal to generate IP packets and sends them to the host network 311.
In order for the splitter 302 to multiply the upstream signal in the time-division manner as described above, it is necessary to coordinate the output timing of the communication data U1, . . . , Un per each of the ONUs 305-1, . . . , 305-n. Here, a distance from the ONUs 305-1, . . . , 305-n to the splitter 302 differ per each of the ONUs 305-1, . . . , 305-n. Therefore, a delay time (signal propagating time) from the ONUs 305-1, . . . , 305-n to the splitter 302 also differs from each other. Therefore, it is necessary to take the difference of the delay times into account in coordinating the signal output timing of each of the ONUs 305-1, . . . , 305-n in order to carry out the time-division multiplication by the splitter 302. For such reason, it is necessary to precisely measure the delay time from each of the ONUs 305-1, . . . , 305-n to the splitter 302 in the PON.
As a method for measuring such a delay time, there has been known a method called as ranging. As a ranging system, there is a system stipulated in ITU-T Recommendation G.983.1 for example (see “ITU-T Recommendation” issued by International Telecommunications Union, January 2005, p. 72, FIG. 25/G983.1-Configuration of the specification points). FIG. 4 is a conceptual diagram for explaining this method and is substantially the same diagram with FIG. 25 in ITU-T Recommendation G.983.1.
In the technology shown in FIG. 4, a processing circuit 402 of the OLT 401 generates and outputs an electrical signal for measuring the delay (referred to as a “ranging signal” hereinafter) at first. The ranging signal is converted into an optical signal by an electrical/optical converter 403 and is sent to an ONU 405 via an optical communication line 404. The inputted ranging signal is converted into an electrical signal by an optical/electrical converter 406 and is inputted to a processing circuit 407. The processing circuit 407 transfers this ranging signal to an electrical/optical converter 408. This ranging signal is then converted again into an optical signal by the electrical/optical converter 408 and is returned to the OLT 401. It is converted into an electrical signal again by an optical/electrical converter 409 and is inputted to the processing circuit 402. The processing circuit 402 measures an elapsed time Tconst from the output to the input of the ranging signal by using a built-in timer not shown.
Here, a delay time when the ranging signal passes through the optical communication line 404 in the downstream direction is the same with that in the upstream direction, such delay time will be defined as Tpd, respectively. Delay times in passing through the converters 403, 406, 408 and 409 will be defined as TiS1, TiO1, TiO2 and TiS2, a delay time when the processing circuit 407 transfers the ranging signal from the optical/electrical converter 406 to the electrical/optical converter 408 will be defined as Ts and an equalized delay time of the processing circuit 407 (i.e., a transmission delay time between the OLT through the ONU) will be defined as Td. Here, TiS1 and TiS2 may be measured or set independently. A sum Tresponce of TiO1, Ts, Td and TiO2 is also measurable. Accordingly, it is possible to obtain the delay time Tpd from Tconst by the following expressions (1a) and (1b):
                                                        Tconst              =                                                TiS                  ⁢                                                                          ⁢                  1                                +                Tpd                +                                  TiO                  ⁢                                                                          ⁢                  1                                +                Ts                +                Td                +                                  TiO                  ⁢                                                                          ⁢                  2                                +                Tpd                +                                  TiS                  ⁢                                                                          ⁢                  2                                                                                                        =                                                2                  ×                  Tpd                                +                Tresponse                +                                  TiS                  ⁢                                                                          ⁢                  1                                +                                  TiS                  ⁢                                                                          ⁢                  2                                                                                        (                  1          ⁢          a                )                                          Tresponse          =                                    TiO              ⁢                                                          ⁢              1                        +            Ts            +            Td            +                          TiO              ⁢                                                          ⁢              2                                      ⁢                                  ⁢                  (                                                    when                ⁢                                                                  ⁢                Td                            =              0                        ,                          Tresponse              =                                                TiO                  ⁢                                                                          ⁢                  1                                +                Ts                +                                  TiO                  ⁢                                                                          ⁢                  2                                                              )                                    (                  1          ⁢          b                )            The splitter 302 can generate the time-division multiplexed upstream signal by finding the delay time Tpd of each of the ONUs 305-1, . . . , 305-n (see FIG. 3) through the procedure described above and by coordinating the output timing of the ONUs 305-1, . . . , 305-n.
As shown in FIG. 3, one OLT 301 can accommodate plural numbers of ONUs 305-1, . . . , 305n in the PON and can additionally accommodate ONUs after initiating the operation of the PON. It is necessary to carry out the ranging as described above for the ONU newly accommodated, when adding the ONU. Beside the case of accommodating the new ONU, there is a case when the ranging needs to be carried out while the PON is in-service. When the ranging of either one ONU is being carried out, the other ONUs are required to stop the upstream communication. It is because reliability of the upstream signal of the other ONUs cannot be guaranteed since the upstream communication of the ranging signal is carried out even though its delay time (see FIG. 4) is not specified, and there is a possibility that the ranging signal collides with the upstream signal of the other ONUs. Therefore, ITU-T Recommendation G.983 and G984 stipulate that the other ONUs should not transmit upstream signals during a ranging period (i.e., a ranging window).
FIGS. 5A and 5B are conceptual diagrams for explaining the ranging operation in the PON. FIG. 5A is a conceptual diagram showing a configuration of the PON and FIG. 5B is a conceptual diagram showing the operation. The splitter and others are omitted in FIG. 5A. FIGS. 5A and 5B show a case when the ranging of the ONU #1 is carried out during when a communication is made between the OLT and the ONU #2 via an optical communication line 501. In this case, it is unable to guarantee the reliability of the upstream communication of the ONU #2 as described above. Therefore, the upstream communication of the ONU #2 is forbidden from the beginning to the end of ranging of the ONU #1 (i.e., from when the OLT transmitted the ranging signal until when it receives the ranging signal). The communication between the OLT and the ONU #2 is started again when the ranging ends (see a, b and c in FIG. 5B).
However, it is not desirable to disrupt the service of the ONU for the ranging from a point of view of assuring quality and reliability of the service. Therefore, it has been desired to provide a countermeasure for carrying out the ranging without disrupting the service to the other ONU.
As a countermeasure for that, there has been a method of providing a buffer on the side of the ONU to temporarily accumulate upstream signals. FIG. 6 is a conceptual diagram showing a case when the buffer is provided in the ONU. As shown in FIG. 6, signals outputted out of a communication terminal are accumulated once in the buffer 604 within the buffer 604. The signals accumulated in the buffer 604 are outputted with timing specified by a read control circuit 605 and arrive at the OLT 601 via an optical communication cable 603.
When no buffer is provided in the ONU, the object ONU of ranging operates as shown in FIG. 7A. That is, this ONU cannot transmit upstream signals inputted from the communication terminal to the OLT during a period corresponding to the ranging window W. Therefore, the upstream signals fall out.
When the buffer is provided in the ONU, the ONU that is not the object of ranging operates as shown in FIG. 7B. The upstream signals inputted from the communication terminal are accumulated once in the buffer and are outputted sequentially with the timing specified by the read control circuit (see FIG. 6). Accordingly, the timing for outputting each upstream signal of the ONU becomes late as compared to the case of FIG. 7A even before the ranging window W is initiated. When the ranging window W is initiated, the read control circuit stops to read the accumulated upstream signals. Therefore, a number of upstream signals accumulated in the buffer increases. After that, when the ranging window W ends, the read control circuit quickly and sequentially reads the upstream signals accumulated in the buffer to output from the ONU.
However, the method of using the buffer leads to an increase of cost of the PON since the required buffer capacity is large. The larger the number of ONUs accommodated in one OLT, the larger the required buffer capacity becomes. Further, the larger the number of splits of the optical communication line (i.e., a number of the splitters) and the larger the transmission band, the larger the buffer capacity becomes.
In addition to that, because the upstream signals need to be quickly read and transmitted when the buffer is provided on the side of the ONU, a high precision and complex control is required.